The invention relates to a receiver for direct sequence spread spectrum code division multiple access signals (DS-CDMA). The invention particularly but not exclusively, relates to such a receiver for receiving GPS signals. In this respect the term GPS is intended to include not only the US Global Positioning System but also the Russian Global Satellite System (GLONASS) and any other equivalent systems which may be established in the future.
The GPS system is a time of arrival positioning system which uses a nominal constellation of 24 low earth orbit satellites to provide a position fix anywhere on the earth""s surface. The satellites broadcast their position and timing information using a direct sequence spread spectrum signal. Currently two frequencies are used in the full system but for low cost civilian use the carrier is at 1575 MHz. The satellites orbits are designed such that provided that there is a clear view of the sky at least five satellites will be in view anywhere at any time. The minimum number of satellites required to be in view for a full position fix is four. These are needed to resolve the three unknown special dimensions and the ambiguity between the receiver clock and satellite clocks.
All of the satellites broadcast on the same frequency using different pseudo random noise (PRN) codes. Thus the down conversion of all the signals can be performed in parallel using a single front end stage. The separation into multiple channels can be carried out at baseband and is normally done in the digital domain.
The GPS signal is a spread spectrum DS-CDMA signal. In order to retrieve the data from this signal it must be correlated with a copy of the PRN code that was used to spread the signal on transmission. This code is known, but the frequency at which the code is being received and the exact timing of the code are not known.
In order to acquire a GPS signal from a satellite, a receiver must identify both the Doppler Shift on the signal and the phase of the satellite PRN code to a high degree of accuracy. If the receiver has tracked the satellite recently, it might be able to estimate the new position of the satellite, its Doppler Shift, and the code phase to be expected. Equally, the receiver may have no information at all with which it can estimate these quantities. In either case, a guess is made at the signal frequency and the code phase. The receiver generates the PRN code with the selected phase and a carrier with the estimated Doppler Shift, and tries to de-spread the signal. If the signal is not found immediately, a search must be performed through code-frequency space until it is.
In general, searches through code-frequency space are performed by searching code space first at a certain frequency. If the signal is not found, a search through code-space begins at a new frequency. As each code phase is tried, the receiver correlates the replica code with the received signal. The output of the correlator is then integrated over a specific period. At the end of this period a decision has to be made whether the code phase was correct or incorrect based upon the value of the integration result. This decision is made by comparing the absolute value of the integration result with a threshold value. If the result is greater than the threshold value, a correct code phase is flagged. If the result is less than the threshold value, the code phase is rejected as incorrect.
Because the correlation process takes place in the presence of noise, the result of the integration will be affected by noise. In some cases, a correct code phase will be rejected as incorrect because the noise superimposed on the integration result brings it below the threshold. In other cases, a xe2x80x98false alarmxe2x80x99 will occur in which an incorrect code phase is flagged as correct because the noise on the integration result has brought it above the threshold. If a lot of false alarms occur, time will be wasted whilst the receiver has to either re-check and reject them , or accepts them and begins to track the signal before losing lock and having to resume the search. If the chance of detecting the correct code is reduced, the receiver will have to either waste time re-checking all the code phases to be certain that the signal is not at that frequency, or will continue searching at a new frequency when the previous one was actually correct.
It follows, then, that the time to acquisition depends upon the probability of detecting the correct code phase and the probability of false alarms occurring. As a result, the choice of value for the decision threshold will have a significant effect on the performance of the receiver when acquiring a satellite in various signal and noise conditions. A well-chosen threshold will provide rapid acquisition in all signal-to-noise conditions which are expected to be encountered.
In Patent Abstracts of Japan, Publication No. 07140224A there is disclosed a spread spectrum signal tracking device in which a spread spectrum signal received by a receiver is inputted to first and second multipliers, where the carrier components of the signal for an I channel and a Q channel are eliminated independently. First and second correlators measure respective correlation values (i) and (q) according to the inputted base band signals of the I and Q channels and a PN code from a PN code generator, and the correlation values are inputted to respective cyclic integrators. The integrators perform addition of the correlation values (I) and (q) according to the number of cycles which is transmitted from a correlation measuring requirement computing portion, and the values are squared by squares and added together through an adder. These signals squared and added together are inputted to a synchronisation recognition portion determine whether or not the signals are in synchronisation with the PN code.
It is further stated that the purpose of the spread spectrum tracking device is to enable spread spectrum signals to be tracked in a shorter time by dynamically determining a correlation measuring time according to the relationship between the signal level of an estimated spread spectrum signal and an anticipated correlation noise level.
It is an object of the invention to enable the provision of a DS-CDMA radio receiver in which the time to acquire or re-acquire a signal from satellite is reduced to a minimum.
The invention provides a receiver for spread spectrum direct sequence code division multiple access signals the receiver including a plurality of correlator channels each of which has a threshold level for determining when correlation between a received PRN code from a given satellite and a locally generated PRN code has been achieved, said receiver further including means for assessing the correlation noise level, means for predicting the signal strength of the signal transmitted by the satellite whose code it is being attempted to correlate and means for setting the threshold level of each of the correlators to a value which is dependant on the correlation noise level and the predicted signal strength.
It has been found that the performance of an acquisition system is significantly affected by the signal-to-noise ratio of the receiver signal. With any particular set of signal and noise conditions, a roughly optimum value may be chosen for the threshold which produces a few false alarms and misses detection very rarely. If the same threshold value is used in a situation where there is more noise, more false alarms are produced and the time to acquisition is increased. If the same threshold value is used in a situation where the signal strength is lower, then the chance of detecting the signal is smaller, and the acquisition may take longer as some correlations might be missed.
It has now been recognised that by adopting a variable threshold level which is dependent on the received signal strength and noise level the time to acquire or re-acquire the signal may be optimised.
The received signal strength may be predicted from a knowledge of the transmitted power of the satellite signal and its distance from the receiver. Provided that the receiver has not moved significantly between the last position fix and the attempt to acquire the satellite signals and has not been switched off for so long that the almanac data is out of date it is possible to predict the positions of each of the satellite visible to the receiver to a reasonable degree of accuracy and hence predict the strength of the signal arriving at the receiver. This, of course, neglects the effects of any obstructions or reflected signals.
The means for assessing the correlation noise level may comprise monitoring means for monitoring the output of at least one of the correlators when that correlator is not producing an output indicating correlation with a transmitted spreading code.
This provides a measure of the correlation noise produced. If the received signal strength can be predicted i.e. the positions of the satellite are known sufficiently accurately with respect to the receiver then this information can be used to set the threshold level to an optimum position which minimises the risk of failing to detect real signals because the correlation is too low and maintains the risk of false detection because of correlation with the noise at an acceptable level.
The means for assessing the correlation noise level may comprise setting the PRN code in at least one of the correlators to one which does not correspond to that of a visible satellite and monitoring the output of that correlator.
In many cases is may be sufficient to assess the correlation noise while starting to attempt to acquire a satellite signal since it is unlikely that there will be an immediate correlation between the locally generated PRN code and that transmitted by the satellite. If, however, it is known, for example from stored almanac data that a particular satellite is not visible to the receiver then by using its PRN code it can be ensured that the output of the correlator is caused solely by noise.
The PRN code may be set so as not to correspond to the code transmitted by any of the satellites. In this case there is no need to know the positions of any of the satellites in order to ensure that any correlation results are produced solely by noise.
For re-acquisition of the signal when the signal from a satellite has been interrupted for less than a given period the predicted signal strength may be set at the signal strength of the received signal prior to the interruption.
When GPS receivers are used as sensors in vehicle navigation systems it is possible that the signals from one or more satellites may be temporarily lost due to passing under trees or passing and more especially passing between tall buildings or passing under or through bridges or tunnels. These interruptions are generally of a fairly short duration as satellites soon come back into view. Their signals do, however, have to be re-acquired and because of the relatively short interruption the signal strength is unlikely to have changed significantly. Accordingly by using the same signal strength for setting the threshold level as was being previously received will generally give optimum threshold value to obtain the fastest time for re-acquisition.
The invention further provides a GPS receiver comprising a receiver as claimed in any preceding claim, said receiver having a plurality of reception channels for locking on to signals from a plurality of satellites, and means for calculating the position of the receiver by processing the signals received from the satellites.
The invention has particular application in GPS receivers where the time to acquire the position (which requires signals to be obtained from at least four satellites; assuming that no other position information is available) is a parameter which is frequently of great importance to the user. Whilst in most cases the signals will be acquired from the satellites in parallel it is necessary to obtain all four signals before the position calculation can be made. Further when acquiring a satellite for the first time any performance gain in ensuring recognition of a correct correlation will be further multiplied by the number of frequencies which have to be searched.